Thrombopoietin is a cytokine consisting of 332 amino acids that increases platelet production by stimulating differentiation and proliferation of hematopoietic stem cells, megakaryocytic progenitor cells and megakaryocytes mediated by its receptor and therefore is promising as a drug for hematological disorders. Recent reports that it stimulates differentiation and proliferation of vascular endothelial cells and endothelial progenitor cells have raised expectations of therapeutic angiogenesis, anti-arteriosclerosis and prevention of cardiovascular events (for example, non-patent document 1, non-patent document 2 and non-patent document 3).
Biologically active substances which have been known so far to regulate platelet production through the thrombopoietin receptor include, in addition to thrombopoietin itself, low molecular weight peptides having affinity for the thrombopoietin receptor (for example, patent document 1, patent document 2, patent document 3 and patent document 4).
As a result of search for nonpeptidic low molecular weight compounds that increase platelet production mediated by the thrombopoietin receptor, low molecular weight compounds having affinity for the thrombopoietin receptor have been reported (for example, patent document to patent document 22).    1) Applications filed by Hokuriku Seiyaku Co., Ltd. relating to 1,4-benzodiazepine derivatives (patent documents 5 and 6)    2) International Laid-open Patent Applications filed by Shionogi & Co., Ltd. (patent documents 7-10)    3) International Laid-open Patent Applications filed by SmithKline Beecham Corp (patent documents 11-19)    4) Japanese Laid-open Patent Application filed by Torii Pharmaceutical Co., Ltd. (patent document 20)    5) International Laid-open Patent Application filed by Roche Diagnostics GMBH (patent document 21)    6) International Laid-open Patent Application filed by Yamanouchi Pharmaceutical Co., Ltd. (patent document 22)
Some reports have been made about pyrazolone compounds (such as non-patent documents 4-13).
Patent Document 1    JP-A-10-72492
Patent Document 2    WO96/40750
Patent Document 3    WO96/40189
Patent Document 4    WO98/25965
Patent Document 5    JP-A-11-1477
Patent Document 6    JP-A-11-152276
Patent Document 7    WO01/07423
Patent Document 8    WO01/53267
Patent Document 9    WO02/059099
Patent Document 10    WO02/059100
Patent Document 11    WO00/35446
Patent Document 12    WO00/66112
Patent Document 13    WO01/34585
Patent Document 14    WO01/17349
Patent Document 15    WO01/39773
Patent Document 16    WO01/21180
Patent Document 17    WO01/89457
Patent Document 18    WO02/49413
Patent Document 19    WO02/085343
Patent Document 20    JP-A-2001-97948
Patent Document 21    WO99/11262
Patent Document 22    WO02/062775
Non-Patent Document 1    Microvasc. Res., 1999: 58, p. 108-113
Non-Patent Document 2    Circ. Res., 1999: 84, p. 785-796
Non-Patent Document 3    Blood 2001:98, p. 71a
Non-Patent Document 4    Huaxue Xuebao (2001), 59(9) p. 1495-1501
Non-Patent Document 5    Synthesis and Reactivity in Inorganic and Metal Organic Chemistry (2000), 30(7) p. 1265-1271
Non-Patent Document 6    Synthesis and Reactivity in Inorganic and Metal Organic Chemistry (2002), 32(4) p. 739-751
Non-Patent Document 7    Synthesis and Reactivity in Inorganic and Metal Organic Chemistry (2002), 32(5) p. 903-912
Non-Patent Document 8    Jiegou Huaxue (2002), 21(5), p. 553-556
Non-Patent Document 9    Polyhedroon (1997), 16(11) p. 1825-1829
Non-Patent Document 10    Arzneim-Forsch (1969), 19(10) p. 1721-1723
Non-Patent Document 11    Structural Chemistry (1999), 10(2), 105-119
Non-Patent Document 12    Chemical Sciences (1996), 51(9), 1240-1244
Non-Patent Document 13    Chemical Sciences (1997), 52(2), 237-242